Quantum entanglement

Quantum entanglement describes the relationship and correlation between two …

Quantum entanglement is one of the most misused concepts around. Entanglement is delicate, rare, and short-lived. At its heart, quantum entanglement is nothing more or less than a correlation between two apparently separate quantum objects. Having discovered that, you might ask "so what is all the fuss about?" The answer lies deep in quantum mechanics.

It is perhaps best to take an example. I shine a laser light through a special crystal. This crystal will occasionally, grab a photon from the light stream and split into two photons with a lower frequency—the sum of the two frequencies adds to that of the original photon—one of the photons is polarized vertically and the other horizontally.

Because we don't know the polarization state and frequency of each photon, we say that they are in a quantum superposition of the two polarization states and all possible frequency combinations. If we were to measure the frequency and polarization of one photon, we would know immediately what the frequency and polarization of the other is. This is because they are linked by the single physical process that generated them. Classically, we could say "aha, these two photons were always in these states, so there is no need to complicate things further."

But, nothing could be further from the truth—well, actually, many things could be further from the truth, but this still ain't true. If I am careful, I can set up my generation process so that it always generates vertically and horizontally polarized photons. And I can pass one of the photons through a device that modifies its polarization. If I then perform measurements on both photons, I will find that the only way to understand the results is that my modification of one photon's polarization state must have also modified the other photon's polarization state.

We know that these photons are not behaving classically but it is stranger than you might think. If these two photons were separated by millions and millions of kilometers, the modification of one photon's polarization state is still balanced by the modification of the other photon's polarization state. This happens because the two photons are a single quantum object—that is, they are described by a single mathematical function that cannot be broken up into separate descriptions for each photon. Furthermore, there is no information exchange involved; the changes do not have to move from one photon to the other, they simply are.

Since I have described something that looks like it can enable long distance communication, lets deal with that as well. It can't. Quantum mechanical measurements are often extremely limited. In this case, we can't ask a photon "what polarization are you?" We can only ask "are you vertically polarized?" The photon's answer will always be yes, or no. But, you cannot know if that is because the other photon has been measured—measuring one photon instantly sets the other photon's polarization in stone—or because it was in a superposition state and you measured first.

The only way to resolve that conundrum is to have a speed-of-light communication channel that says something like "measure now." In which case, you might as well just send the all the information over the speed-of-light channel.

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.